Telomeres shorten dramatically with human aging and during human carcinogenesis due to insufficient levels of telomerase, the enzyme that synthesizes telomere repeats. This progressive telomere shortening can exert profound effects on tissue homeostasis in both human and mouse, principally in organs with high renewal requirements. In telomerase knockout mice, telomere dysfunction induces high rates of apoptosis and impairs tissue maintenance. The precise mechanisms by which telomere shortening causes these deleterious effects and the specific cell compartments within a tissue that are principally impaired by telomere dysfunction remain to be defined. Telomere shortening is a hallmark of human carcinogenesis, indicating that early stages of tumor development occur in a setting of insufficient levels of telomerase. We have shown that critical telomere shortening in telomerase-deficient, p53-mutant mice leads to cycles of chromosomal fusion-bridge-breakage. This process leads to spontaneous formation of cancer types that resemble those of aging humans - carcinomas - and to the classical chromosomal changes seen in human carcinomas, including non-reciprocal translocations and gene copy number changes. Here, we propose that the mechanism by which telomere dysfunction impairs tissue function is principally by impairing tissue self-renewal and stem/progenitor cell function. Although telomerase is reactivated in 90% of human cancers, our understanding of the advantages conferred by telomerase in tumor progression is incomplete. We have developed a sophisticated mouse model system to directly test these hypotheses using a novel allele of TERT, the catalytic component of telomerase that can be reactivated. This experimental design enables us to study the effect of telomere dysfunction on tissue maintenance, stem/progenitor cell function and on cancer initiation. Furthermore, it provides us with the unique opportunity to reactivate TERT to determine if it can reverse the defects associated with telomere shortening and promote tumor progression. The aims of the proposal are: (1) To understand how TERT deficiency affects telomere maintenance, chromosomal stability, and apoptosis and p53 responses in epithelial tissues in vivo (2) To ascertain how telomere dysfunction affects mammary epithelial development and stem/progenitor cell function and (3) To determine how TERT reconstitution affects tumor progression and metastasis in a breast cancer-specific model driven by telomere instability.